Hitherto, various types of actuators (driving devices) have been developed depending on applications. Attention has recently been focused on an artificial actuator taking into account the mechanism of muscles of an organism, which is generally called an “artificial muscle”.
In an actual organism, an articulation or the like is formed by a pair of a muscle called “agonist” and a muscle called “antagonist”. For example, when the articulation is moved, one muscle is contracted while the other muscle is maintained in the relaxed state. Because the other muscle in the relaxed state can change its extended or contracted state by a small force, an amount of work generated by the contraction of the one muscle can be substantially all used to move the relevant articulation (i.e., to perform external work). If the other muscle is not maintained in the relaxed state and a large force is required to extend the other muscle, the amount of work generated by the contraction of the one muscle is consumed in its large part for work to extend the other muscle, and work efficiency is reduced. Accordingly, it is preferable for the actuator, which is to be used as the artificial muscle, that when the actuator is not driven, a relative positional relation of the actuator can be freely changed by a small force, namely a static force is as small as possible.
As a known actuator capable of extending and contracting along a predetermined axis of movement, there is disclosed an arrangement in which two members are contacted with each other and traveling waves are applied to a contact surface between the two members by utilizing ultrasonic waves, thus causing the two members to move relatively (see, e.g., Japanese Unexamined Patent Application Publication No. 8-182352 (Patent Document 1), Japanese Unexamined Utility Model Registration Application Publication No. 2-129193 (Patent Document 2), and Japanese Unexamined Patent Application Publication No. 2-070274 (Patent Document 3), etc.). Also, Japanese Unexamined Patent Application Publication No. 2007-181384 (Patent Document 4) discloses an arrangement in which a displacement is generated by applying asymmetrical motions to a shaft so as to provide frictional forces asymmetrical to each other.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 8-182352
Patent Document 2: Japanese Unexamined Utility Model Registration Application Publication No. 2-129193
Patent Document 3: Japanese Unexamined Patent Application Publication No. 2-070274
Patent Document 4: Japanese Unexamined Patent Application Publication No. 2007-181384
With any of the arrangements disclosed in the prior-art documents mentioned above, however, because a frictional force is basically utilized as a primary driving source, a static frictional force acting upon the contact surface between the two members needs to be comparatively large. Thus, because a comparatively large driving force is required to move the two members relative to each other, it has been impossible in principle to realize the above-described relaxed state of the muscle. Further, when the actuator is driven, a relatively large driving force needs to be generated against the static frictional force, and the comparatively large driving force necessarily causes a wear between the two members. This results in another problem that the life span of the actuator itself cannot be prolonged.